1. Field of the Invention
The present invention relates to a vibratory separator, a screen assembly for a vibratory separator and a method for fitting a screen assembly into a vibratory separator.
2. Description of Related Art
In the drilling of a borehole in the construction of an oil or gas well, a drill bit is arranged on the end of a drill string and is rotated to bore the borehole. A drilling fluid or “drilling mud” is pumped through the drill string to the drill bit to lubricate the drill bit. The drilling mud is also used to carry the cuttings produced by the drill bit and other solids to the surface through an annulus formed between the drill string and the borehole. The drilling mud often contains expensive synthetic oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires the solids to be removed from the drilling mud. This is achieved by processing the drilling fluid. The first part of the process is to separate the solids from the solids laden drilling mud. This is at least partly achieved with a vibratory separator, such as those shale shakers disclosed in U.S. Pat. No. 5,265,730, WO 96/33792 and WO 98/16328.
Shale shakers generally comprise an open bottomed basket having one open discharge end and a solid walled feed end. A number of rectangular screens are arranged in the basket, which are held in C-channel rails located on the basket walls, such as those disclosed in GB-A-2,176,424. The basket is arranged on springs above a receptor for receiving recovered drilling mud. A skip or ditch is provided beneath the open discharge end of the basket. A motor is fixed to the basket, which has a drive rotor provided with an offset clump weight. In use, the motor rotates the rotor and the offset clump weight, which causes the basket and the screens fixed thereto to shake. Solids laden mud is introduced at the feed end of the basket on to the screens. The shaking motion induces the solids to move along the screens towards the open discharge end. The recovered drilling mud is received in the receptor for further processing and the solids pass over the discharge end of the basket into the ditch or skip.
The screens are generally of one of two types: hook-strip; and pre-tensioned.
The hook-strip type of screen comprises several rectangular layers of mesh in a sandwich, usually comprising one or two layers of fine grade mesh and a supporting mesh having larger mesh holes and heavier gauge wire. The layers of mesh are joined at each side edge by a strip which is in the form of an elongate hook. In use, the elongate hook is hooked on to a tensioning device arranged along each side of a shale shaker. The shale shaker further comprises a crowned set of supporting members, which run along the length of the basket of the shaker, over which the layers of mesh are tensioned. An example of this type of screen is disclosed in GB-A-1,526,663. The supporting mesh may be provided with or replaced by a panel having apertures therein.
The pre-tensioned type of screen comprises several rectangular layers of mesh, usually comprising one or two layers of fine grade mesh and a supporting mesh having larger mesh holes and heavier gauge wire. The layers of mesh are pre-tensioned on a rigid support comprising a rectangular angle iron frame and adhered thereto. The screen is then inserted into C-channel rails arranged in a basket of a shale shaker. An example of this type of screen is disclosed in GB-A-1,578,948.
A further example of a known rigid support is disclosed in PCT Publication No. WO 01/76719, which discloses, among other things, a flat panel like portion having apertures therein and wing portions which are folded to form a support structure, which may be made from a single sheet of material. This rigid support has been assigned the Trade Mark UNIBODY by the applicants.
The layers of mesh in the screens wears out frequently and therefore needs to be easily replaceable. Shale shakers are generally in the order of 5 feet wide and 10 feet long. A screen of dimensions 4 feet wide by 10 feet long is difficult to handle, replace and transport. It is known to use two, three, four or more screens in a single shale shaker. A standard size of screen currently used is of the order of 4 feet by 3 feet.
U.S. Pat. No. 4,582,597 discloses a vibratory separator having a basket and channels arranged on internal faces of the basket for receiving the two sides of a screen assembly. An inflatable hose is arranged in the channels. The screen assembly comprises screening mesh laid over and fixed to a frame. The screen assembly is slid into the channels in the vibratory separator. The hose is inflated using a pneumatic fluid to force the frame of the screen assembly to assume a crowned center, in order to reduce whipping.
GB-A-2,176,424 discloses a vibratory separator having a basket and channels arranged on internal faces of the basket for receiving a screen assembly and a clamping device. The screen assembly comprises screening mesh laid over and fixed to a frame. The clamping device comprises a frame of similar dimensions to the frame of the screen assembly. The frame has an inflatable stocking thereon, which is attached to pneumatic or hydraulic fluid supply means. When both the screen assembly and the clamping device are slid into the channels, the stocking is inflated to fix the screen assembly in the channels. There is also disclosed a clamping device having an intermediate frame member on which a tributary of the stocking is arranged. Support members are provided above and below the screen assembly so that, in use the tributary is inflated to clamp the screen assembly between the supports.
GB-A-2,176,425 discloses a vibratory separator having a basket and channels arranged on internal faces of the basket for receiving a screen assembly. The screen assembly comprises screening mesh laid over and fixed to a frame. The channels have an inflatable stocking therein for clamping the frame of the screen assembly in the channels. The screen assembly also has a stiffening rib along each of two intermediate parts of the screen assembly and the vibratory separator has a tributary of the stocking along each of two intermediate parts of the vibratory separator, such that in use, the tributaries are inflated to clamp on the stiffening ribs.
U.S. Pat. No. 4,809,791 discloses a shaker having a screen. The screen is supported on flexible transverse arch-shaped beams attached to the sides of a frame. A ram is arranged in the shaker and is attached to the center of the flexible transverse arch-shaped beam. The ram is extended to increase the tension in the screen.